(1) Field of the Invention
The present invention relates to a method and apparatus for controlling a heater for heating an air-fuel ratio sensor, such as an O.sub.2 sensor. Such a sensor is used in an internal combustion engine for measuring the air-fuel ratio in the exhaust gas.
(2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (for example, an O.sub.2 sensor) system, a base fuel amount is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount is corrected by an air-fuel ratio coefficient which is calculated in accordance with the output signal of an air-fuel ratio sensor for detecting the concentration of specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichometric air-fuel ratio.
Note, an output signal of an oxygen-battery-type O.sub.2 sensor, which shows a rich or lean air-fuel ratio, is stable when the element temperature of the O.sub.2 sensor is higher than a definite value. That is, the O.sub.2 sensor is in an inactive state when the element temperature thereof is lower than a definite value, and the O.sub.2 sensor is in an active state when the element temperature thereof is higher than a definite value. As a result, the O.sub.2 sensor has a limited application. According by, when the O.sub.2 sensor is in an active state, it is possible to distinguish whether the air-fuel ratio is rich or lean, by comparing the output voltage of the O.sub.2 sensor with a definite value, such as 0.45 V.
In order to keep the O.sub.2 sensor in an active state, the O.sub.2 sensor in which a heater is incorporated is already known. In the above mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed in the exhaust gas flow passage, the heater is turned ON and OFF in accordance with a driving condition parameter of the engine, such as the engine rotational speed or a load of the engine, such as the amount of intake airflow.
Nevertheless, when the heater is turned ON and OFF in accordance with the driving condition parameter of the engine, in other words, when the heater is turned ON or OFF in accordance with driving condition parameters, the heater is frequently switched from ON to OFF or vice versa, when the engine is driven at the boundary of a heater ON area and a heater OFF area determined by the driving condition parameter, or when the gear of the engine is changed. If the heater is frequently switched from ON to OFF or vice versa, so that the number of times that the heater is switched exceeds five hundred thousand within a lifetime of the engine, the wiring of the heater may be damaged beyond repair.
When the heater wire is disconnected, the air-fuel ratio control according to the output signal of the O.sub.2 sensor is activated, even though the O.sub.2 sensor is in an inactive condition, thus impairing the driveability, the emission characteristics, and the fuel consumption.